The present invention relates to microwave device integration.
At higher microwave frequencies (also known as millimeter frequencies, i.e. above 30 GHz up to about 300 GHz or higher) it is extremely difficult to achieve a compact source of rf power. Transistors of any sort are difficult enough to construct with even small-signal gain in the higher frequencies of this regime, and transistor oscillators are simply not capable of providing the needed power at higher frequencies. Thus, to provide an initial source of rf power (which can then be modulated, phase-shifted, attenuated, etc.), microwave diodes (such as IMPATTs, TRAPATTs, BARITTs, TUNETTs, or others) appear to be the only solid-state option for the foreseeable future.
However, integrating microwave diodes into functional circuitry for near-millimeter operation presents major problems, at which the present invention is aimed.
In common practice, microwave diodes are produced in discrete form rather than as integrated circuits, so that the device can be bonded to a good heat sink. (The power density dissipated in an advanced microwave diode is very high, particularly at higher microwave frequencies, and thermal coupling becomes a crucial design limitation. Gallium arsenide or other III-V substrates tend to be rather poor thermal conductors.) Heat sink materials are usually (preferably) highly conductive metals which also form the ground contact of the diode. Thus, in the prior art, a discrete semiconductor microwave diode would be assembled and bonded to a metal heat sink, and would then be bonded to connect it to impedance matching passive elements off-chip.
However, this prior art approach has tremendous disadvantages. First, the bond-wire connections introduce unpredictable parasitic reactances, so that the resonance characteristics of a completed structure are not predictable in either center frequency or Q. Thus much more complicated circuit designs must be used at the module level to compensate for this unpredictability, and such expensive techniques as hand-adjustment of matching values and selection of matched sets of components must be used extensively. Second, the amount of hand labor required to produce a functional device is much greater than for any monolithic structure. Third, yield is greatly degraded, because the step of affixing a bond wire to a thin semiconductor chip is very likely to break the chip under pressure. Fourth, this major source of yield loss occurs at a very advanced stage of processing, i.e. the devices being destroyed are nearly completed, and thus their destruction is a greater loss than the destruction of an equal number of devices early in processing would be. Fifth, a particular difficulty of the prior art is that efficiently combining multiple diodes to achieve greater power output becomes even more difficult.
Many of these difficulties are inherent in the discrete device assembly process, and could be avoided if it were possible to build monolithic IMPATT diode structures. However, monolithic fabrication of these diodes for operation at higher microwave frequencies has heretofore not been possible, due to the incompatible processing requirements of heat sink materials and of on chip impedance-matching circuits. That is, the heat-sinking requirements of the active area demand that the active area be thermally extremely closely coupled to the heat sink, and in practice this means that the active area must be physically located on the heat sink. However, the passive elements for impedance-matching must not be located so close to the heat sink (if the heat sink is conductive), or, at higher microwave frequencies, it will be impossible to configure an inductor: the parasitic capacitance to the substrate will make every element look like a capacitor. However, use of non-conductive heat sinks, such as diamond, is not practical for two terminal devices, such as IMPATT diodes, which require a good backside contact. Thus, there is an inherent conflict between the needs of the active elements and passive elements which has heretofore precluded their integration in a single monolithic structure.
In some approaches previously considered, the thermal problem was addressed either by placing the heat sink metal inside fine vias produced in the GaAs substrate or by spreading the device area over a large surface and using the GaAs substrate as the heat sink. In both approaches the impedance matching circuitry is produced on the semi-insulating part of the substrate. However, this is not optimal, because the heat sink capabilities of such configurations are limited, while the processing depends on the use of expensive technology.
An attempt was made at RCA laboratories to fabricate monolithic IMPATT diodes on silicon substrates (RCA Review, vol. 42, p. 633 (1981)). The method described makes use of selective etching of silicon and ion implantation through the thin portions of the wafer to achieve device active layers. Metallization of via holes and production of impedance matching circuitry on the silicon substrate was mentioned, but practical devices were not demonstrated. It is believed that the silicon slice could not be made resistive enough after all necessary implantations. No complete usable method of producing monolithic circuits was presented. Although silicon does have better thermal properties than gallium arsenide, this particular approach appears impracticable due to the other shortcomings of silicon as a substrate.
The present invention, in various embodiments, teaches the fabrication of monolithic microwave diodes, such as IMPATT diodes, produced on large-area metallic heat sink material which also serves as the ground terminal of the device. (The heat sink is plated onto the topside of the active layer of a chip, and the original semi-insulating substrate is then etched off.) The device protective packaging, device-to-circuit transitions and the impedance matching circuits are all produced with the use of varying thicknesses of coated-on dielectric (preferably polyimide) layers which are produced on the surface of the heat sink substrate. In this way, the polyimide layers become an integral part of the device, allowing the fabrication of monolithic circuits without the thermal disadvantages of a gallium arsenide substrate. FIGS. 8 and 9 show sample implementations of this concept. Polyimide is used as the dielectric medium over which impedance matching networks, transmission lines, and bias filters are produced. Electrical contact between the circuitry on polyimide and the IMPATT diode is provided by via holes in the polyimide layer directly above the diode. In this fashion the device is encapsulated inside a stable and durable dielectric, while device-to-circuit parasitics are reduced considerably, since the on-chip circuits are placed directly above the device. The thickness of the polyimide layer is typically 10 to 50 micrometers, the actual thickness depending on the particular application. The total device height, for an IMPATT to operate at the higher microwave frequencies of most interest, must not be more than a few micrometers. This is achieved by using a selective etching technique.
Thus, the present invention provides a fundamentally new structure for microwave devices. The use of a multilayer structure of polyimide (or other coated-on dielectric) over a metallic heat-sink ground plane permits the mounting of both active area and of the matching passive elements to be optimized. Moreover, the polyimide provides a substrate of good enough quality that fairly conventional integrated circuit patterning and fabrication techniques can be used to build elements with high reproducibility.
Most advantageously, the present invention is used to combine any desired number of diodes having a common semiconductor layer structure with any desired combination of passive elements. Thus, power combining networks can be used to match multiple diodes; filter networks can be combined monolithically with a diode oscillator; or bias control networks for a VCO can be integrated with a diode oscillator. It should also be noted that the present invention does not preclude use of three-terminal active devices either, although two-terminal devices are easier to use with present invention.
The fabrication scheme used in the present technique minimizes the transition losses between elements, since inductive elements are produced on the second level and connections are made by vertical via holes directly above capacitors.
Another important advantage of the present invention is that it permits the manufacturing economies of fabricating a whole wafer of devices at a time to be applied to the processing of millimeter-wave IMPATTs, which has not heretofore been possible.
All capacitors are produced simultaneously on a thin layer of polyimide. Similarly, all inductors are produced simultaneously on a thick layer of polyimide. This means that for a given mask set, all capacitor and inductor values are proportional. Absolute values of capacitors and inductors are determined by relative thicknesses of polyimide layers which are produced separately. By varying the thicknesses of the two polyimide layers in proportion to each other, the center frequency of the impedance matching network can be adjusted over a large fraction of the bandwidth without significantly altering the circuit properties seen by the active device.
The thermal properties of a microwave diode can in general be improved if the active area of the device is spread over a larger area. That is, for a given total active area and substrate/heat-sink structure, the active area can dissipate higher power if it is configured as many separate small pieces, since the thermal spreading resistance of the substrate/heat-sink around the separate portions of the active area will lower the total thermal resistance seen by the total active area. Thus, for a given total active area it is desirable to use area-spreading techniques to get maximum power. However, this implies that the individual active area portions must be smaller, and this is a problem when using the techniques taught by the prior art. High frequency microwave devices are typically a few thousandths of an inch in diameter if produced in one piece. These small dimensions already impose severe limitations on the conventional processing technology. Spreading the active area of the device means that each piece of the device will be even smaller in area, which increases the difficulty of fabricating. However, the monolithic technology of the present invention is capable of processing much smaller geometries (accuracy down to 1 micron). Therefore the need, resulting from area spreading, for smaller area devices is not as severe a limitation as the case with conventional technology. The smallest piece of an active device can be at least one order of magnitude smaller than what is achievable today, and therefore are spreading is very greatly facilitated. Thus, the present invention provides greater area spreading (and therefore greater output power capability) for a given total active area.
The total area of an active device is determined by the external circuit capabilities. The maximum device area, and hence the output power, is determined (in the prior art) by the minimum circuit impedance levels which can be achieved by the external circuitry. Therefore, for a given circuit configuration the output power per device is fixed. However, in addition to the advantages of the present invention in area spreading, the minimum impedance level problem is also greatly mitigated by the present invention, since at least some impedance matching can be accomplished within the monolithic package. That is, parallel-connected IMPATT diodes having a net impedance of (for example) one quarter ohm could be impedance-transformed, using a passive-element network within the monolithic element, up to a level, such as ten or more ohms, which is easily matched to external impedance levels.
Output powers of several devices can be combined at the circuit level to increase total output power. (Power combining, as distinguished from area spreading, is usually used in the art to refer to this configuration, i.e. to combination of devices in the external circuitry as opposed to use of multiple active areas within a single device package.) While the present invention is particularly advantageous in solving the problems of area spreading, it is also advantageous in facilitating power combining, since the problems of matching to and bonding to the device package are so greatly reduced.
The capability of handling smaller area devices also implies that the monolithic technology of the present invention becomes even more advantageous as frequency is increased further. This is because the device sizes must be reduced as the frequency increases to keep device impedance levels constant. (Transit times decrease as the frequency increases. Drift region thicknesses decrease and consequently the capacitance per unit area increases. Reactive impedance equals the inverse of two pi times frequency times capacitance, and therefore admittance per unit area increases with frequency.) Above 100 GHz, the device sizes are prohibitively small (about 20 microns) for conventional technology, whereas the limitations on the monolithic technology of the present invention are not reached until above 300 GHz.
Another one of the difficulties of the prior art at high frequencies is to transform circuit impedances down to the device impedance levels. Two terminal devices such as IMPATTs generally produce larger output powers when the device impedance is allowed to go down, because the device negative resistance decreases sharply as the signal voltage grows in magnitude. The signal grows until the device negative resistance is equal to the external circuit resistance. In other words, the larger signal values will be obtained for smaller negative resistance conditions. One way to assure that the signal voltage grows to reasonable values is to restrict the device area so that the small signal impedance levels will be high. This of course reduces the power output capability of the diode. Device impedance levels less than one ohm are desirable, but are seldom reached in conventional technology due to manufacturing difficulties and the very complex circuit configurations required to compensate for parasitic circuit elements. The monolithic technology of the present invention overcomes these difficulties. The external elements are of conventional microstrip and lumped element type. Furthermore, the parasitic elements are almost totally eliminated, so that circuit impedance transformation can be achieved more easily and reproducibly.
Another advantage of the monolithic technology is the reproducibility achieved from device-to-device and from wafer-to-wafer. IMPATT diodes have negative conductances over a large frequency range, so that the diodes themselves are not inherently very frequency-selective, and the oscillation frequency is determined by the external circuit elements. Monolithic technology ensures that all circuit elements are nearly identical. This point has been demonstrated experimentally. In a 60 GHz design, a 1.6% variation in oscillation frequency under identical operating conditions was achieved over a 6 cm2 wafer--about 400 devices. Yields are also very high as expected, typically 90%. This means that oscillators constructed according to the present invention will have an operating frequency as-fabricated which can be much more accurately predicted, so that much less hand-tuning and selection of matching components is necessary to configure good working systems.
One of the unique teachings of the present invention is that contact to the semiconductor diode elements is not made by external leads, but is made by deposited thin-film metallization. This point of novelty itself is a source of numerous advantages.
According to the present invention there is provided: A microwave device comprising: a metal heat sink; at least one semiconductor active element located on said metal heat sink; a coated dielectric overlying said semiconductor active device element, said coated dielectric containing at least one via hole above said active element; a thin film metallization pattern on said coated dielectric; and bond pad areas on said coated dielectric, said bond pad areas being connected by said thin film metallization through said via hole to said active element.
According to the present invention there is provided: A microwave device comprising: a metal heat sink; a plurality of semiconductor active elements located on said metal heat sinks; a coated dielectric overlying said semiconductor active device element, said coated dielectric containing a plurality of via holes above said active elements, and said dielectric laterally separating said semiconductor active elements so that said active elements are not within a common continuous body of semiconductor material; a thin film metallization pattern on said coated dielectric; and bond pad areas on said coated dielectric, said bond pad areas being connected by said thin film metallization through said via hole to said active element.
According to the present invention there is provided: A microwave device comprising: a metal heat sink; at least one semiconductor active element located on said metal heat sink; an organic dielectric overlying said semiconductor active device element, said dielectric containing at least one via hole above said active element; a thin film metallization pattern on said dielectric; and bond pad areas on said dielectric, said bond pad areas being connected by said thin film metallization through said via hole to said active element.
According to the present invention there is provided: A microwave device comprising: a metal heat sink; at least one semiconductor active element located on said metal heat sink; first and second layers of coated dielectric overlying said semiconductor active device element, said first layer of coated dielectric containing at least one via hole above said active element; thin film metallization on said coated dielectric layers; said thin film metallization defining capacitor plates on said first dielectric layer; bond pad areas on one of said dielectric layers, said bond pad areas being connected by said thin film metallization through said via hole to said active element.